Mongol conquest of Khwarezmia

The Mongol conquest of Khwarezmia occurred from 1219 to 1221 when the Mongol Empire under Genghis Khan conquered the Central Asian state of Khwarezm. The Mongols reduced a prosperous region of 2 million inhabitants to a devastated region of 200,000 residents in just two years, and it marked the first Mongol conquest in the west.

Background
In 1205, the Naiman prince Kuchlug was forced to flee from Mongolia after Genghis Khan and his Mongol Empire conquered their lands. In 1208, Genghis Khan defeated Kuchlug and forced him to flee to the Western Liao, marrying the daughter of Emperor Zhilugu. Shortly after, Zhilugu rebelled against his father-in-law and overthrew him, taking control of Western Liao. In 1216, Kuchlug attacked the city of Beshbalik, which was under Mongol protection, so Genghis Khan sent his general Jebe to defeat Kuchlug at Balasaghun. Within two years, the entirety of the Western Liao was conquered by the Mongols, who now bordered the Khwarezmian empire under Shah Muhammad II of Khwarezm. Genghis Khan sent traders to Khwarezm to establish trade, but the Governor of Otrar, Inalchuq, had the traders imprisoned after accusing them of being spies. Genghis Khan then sent three ambassadors to the Shah to demand the merchants' release, and the merchants and one of the ambassadors were executed. Genghis Khan was angered at this breach of hospitality, so he gathered information from the Silk Road about his enemies, assembled siege engineers from China, and separated his army into three columns for an invasion of Khwarezm.

War
The Mongolians had 100,000 troops to Khwarezm's 60,000, and, in the winter of 1219, Jochi and Jebe were sent to ravage the Ferghana Valley with 20,000 troops. The Khwarezmians believed this invasion to be the main force, but Chagatai Khan and Ogedei Khan passed through the Dzungarian Gate and besieged the 20,000-strong garrison of Otrar. After five months, a deserter opened the gates, allowing the Mongols into the city; all of its inhabitants were massacred or enslaved, and it was razed to the ground; Inalchuq was executed by having silver poured into his eyes and ears. Genghis Khan and his army crossed the impassable Kyzylkum Desert and besieged Bukhara, whose defenders attempted to sally out before being massacred in an open battle. Genghis Khan now marched on the Khwarezmian capital of Samarkand in 1220, assaulting the city and its 40,000-strong garrison. On the third day, as the defenders sortied, Genghis Khan launched a feigned retreat, luring out and massacring half of the garrison. Two relief attempts failed, and, on the fifth day, all but a handful of soldiers surrendered. The 100,000 residents of the city were slaughtered, and the Shah and his son escaped to the west, leading to Genghis Khan sending Subedei and Jebe to take 20,000 troops west to track them down. After Samarkand's fall, Genghis Khan besieged Urgench, and Jochi was removed from command for refusing to sack the city; he was replaced by Ogedei, who, in turn, had the city destroyed. While Urgench was being destroyed, Tolui Khan and 50,000 troops invaded Khorasan, razing Balkh, Merv, and Nishapur in rapid succession and sparing Herat and other cities which peacefully surrendered. The Mongols used practical brutality to subjugate their subjects, preventing resistance to their rule.

Jalal ad-Din's revolt
Muhammad's son Jalal ad-Din Mingburnu recruited an army of 60,000 Turkic and Afghan warriors, leading to Genghis Khan sending 30,000 troops under the Tatar nobleman Shikhikhutug to attack Jalal ad-Din at Parwan to the north of Kabul. The ensuing battle saw the two sides meet in a narrow valley unsuitable to the Mongolian cavalry. The Mongol army was dealt a humiliating first defeat at the hands of a foreign foe, and Jalal ad-Din had his men counterattack as the Mongols retreat; half of them were lost, while the other half escaped. The defeat broke the illusion of Mongol invincibility, leading to Khwarezmian rebellions in Central Asia. However, Jalal ad-Din's army fell into discord over the next few months, and he fled to India to seek refuge. Genghis Khan caught up to Jalal ad-Din before he could cross the Indus River, and the Mongols outflanked the Khwarezmians and destroyed their army. 20,000 Mongolian troops were then sent to chase down the prince, but he was nowhere to be found. Most of Khwarezm was annexed, and the Shah died in exile on an island in the Caspian Sea. The Mongols reduced the population of Central Asia to 200,000 from 2 million,